The present invention relates generally to an ionization monitor and more particularly to an ionization monitor having an improved very high megohm value resistor.
Ionization monitors are used to detect the presence of low levels of radioactivity in gas streams. Such monitors may include a counting chamber for containing the gas under test, such as a pipe through which the gas is constantly flowing. If ionizing radiation is present, ionic and electronic currents are caused to flow between a counting electrode and the wall of the chamber.
The problem solved by this invention is the measurement of these extremely small currents. Ionization monitors may include an electrometer circuit including 100% negative feedback for amplifying these currents and converting them to measureable voltages. The negative feedback is typically provided by a resistor, a device that linearly converts current to voltage. Unfortunately, for measurement of currents on the order of a few electrons per second, a very high impedance resistor is needed.
Conventional high impedance resistors are made of composites including carbon. These resistors provide a maximum resistance on the order of 10.sup.14 -10.sup.15 ohms, a value too low to enable the detection of extremely low samples of radioactivity in a gas. Accordingly, the invention was developed to provide a maximum resistance on the order of 10.sup.17 -10.sup.18 ohms to permit the detection of as little as 1 radioactive event per second in a gas sample.